Compact, high voltage capacitors are utilized as energy storage reservoirs in many applications, including implantable medical devices. These capacitors are required to have a high energy density, since it is desirable to minimize the overall size of the implanted device. This is particularly true of an Implantable Cardioverter Defibrillator (ICD), also referred to as an implantable defibrillator, since the high voltage capacitors used to deliver the defibrillation pulse can occupy as much as one third of the ICD volume.
Electrolytic capacitors are used in ICDs because they have the most nearly ideal properties in terms of size, reliability and ability to withstand relatively high voltage. Conventionally, such electrolytic capacitors include an etched aluminum foil anode, an aluminum foil or film cathode, and an interposed kraft paper or fabric gauze separator impregnated with a solvent-based liquid electrolyte. While aluminum is the preferred metal for the anode plates, other metals such as tantalum, magnesium, titanium, niobium, zirconium and zinc may be used. A typical solvent-based liquid electrolyte may be a mixture of a weak acid and a salt of a weak acid, preferably a salt of the weak acid employed, in a polyhydroxy alcohol solvent. The electrolytic or ion-producing component of the electrolyte is the salt that is dissolved in the solvent. The entire laminate is rolled up into the form of a substantially cylindrical body, or wound roll, that is held together with adhesive tape and is encased, with the aid of suitable insulation, in an aluminum tube or canister. Connections to the anode and the cathode are made via tabs. Alternative flat constructions for aluminum electrolytic capacitors are also known, comprising a planar, layered, stack structure of electrode materials with separators interposed therebetween, such as those disclosed in the above-mentioned U.S. Pat. No. 5,131,388.
Stacked electrolytic capacitors are typically constructed with a plurality of anodes and cathodes, which must be separated by a liquid absorbent insulative material, and are impregnated by an electrically conductive electrolyte. If the separator is not present as a line of sight barrier between any anode and adjacent cathode, there exists a danger of physical contact, as well as electrical breakdown of any incidental gasses present in the completed capacitor. Either of these scenarios would result in an undesirable partial or complete discharge event with a high probability of device failure.
Stacked electrolytic capacitors are typically insulated from their surrounding metal case using a boot or Kapton tape. If the separator and/or boot/Kapton tape is not present as a line of sight barrier between any anode and adjacent cathode space or case, there exists a danger of physical contact, as well as electrical breakdown of any incidental gasses present in the completed capacitor due to the close tolerances requisite to achieve a compact part. Either of these scenarios would result in an undesirable partial or complete discharge event, with a high probability of device failure.
Stacked high voltage electrolytic capacitors often utilize physical features in the constituent components of assembly with the aim of assuring precision of physical alignment such that the dimensions of those components leave physical margins that assure adequate separator coverage between all anodes and cathodes. The anodes also have a minimum distance between the anodes and the case wall, which prevents complete packaging efficiency utilization.